1. Field of the Invention:
The present invention relates to a contact detecting apparatus for detecting the contact of a tool with a workpiece when they are moved relatively to one another.
2. Description of the Prior Art:
A conventional contact detecting apparatus is provided with a coil mounted on the end of a spindle head in which a spindle is rotatably received. This coil is connected to an AC power source to be energized, so that the coil generates loops of magnetic flux surrounding the spindle which induces current in an axial direction of the spindle. When a tool does not contact a workpiece, a looped secondary circuit connecting the spindle and a machine body is not completed, whereby there is no induced current. On the other hand, when the tool contacts the workpiece, the looped secondary circuit is completed to produce induced current. In this way, the secondary circuit is completed depending upon the contact between the tool and the workpiece, and an exciting current in the coil on the primary side is changed depending upon the change in electric current in the secondary circuit. Accordingly, the contact between the tool and the workpiece is detected by connecting the coil and a resistance in series and detecting the change in voltage across the resistance by a voltage detector.
In such an arrangement, the coil corresponds to a primary winding of a transformer, and the looped secondary circuit constitutes a secondary winding of one turn. The winding ratio of the secondary winding to the primary winding is very small, so that the change in the exciting current in the primary winding is very small, even if there is an ON-OFF change in the secondary circuit.
Furthermore, it is extremely difficult to produce a large induced current in the secondary circuit with light contact between a sharp edge of the tool and the workpiece, whereby it is difficult to improve the detecting sensitivity.